1. Field of the Invention
The present invention relates to an electric variable resistor with a switching mechanism having a resistive element with a layered structure. More particularly, the present invention has a switching function and a variable electric resistor in which a brush slides on a resistive element which is used, for example, as a throttle valve opening sensor for electric apparatus and vehicles.
2. Description of the Prior Art
Generally speaking, in a variable resistor of the present kind, a brush slides on a resistive element from one end to the other end thereof. A variable resistor of this kind may be incorporated in a control system in which the rotating range, for example detecting a throttle valve opening value, defined by the pitch between ends of the resistive element coincides with the rotating range of a mechanism to be controlled.
A conventional or similar throttle opening sensor is shown, for example, in Japanese Utility Model Laid-open Print No. 62(1987)-81004 published on May 23, 1987 without examination.
FIG. 1 shows a front view of the base plate of the conventional type throttle valve opening sensor. FIG. 2 shows a partial enlarged plan view of the FIG. 1.
Referring to the FIG. 1, the conventional throttle valve opening sensor basically includes a base plate (50), a plurality of electrodes (51a,51b) and resistive elements (52a,52b) with a layered structure. The electrodes (51a, 51b) and the resistive elements (52a,52b) are formed on the base plate (50). A brush (54) (shown in FIG. 2) is directly in contact with the surface of the electrodes for detecting a throttle opening value. The resistive elements (52a,52b) are independently of the electrodes (51a,51b) formed on the surface of the base plate (50). Protective layers (53a,53b) are formed on the surface of the base plate (50). The protective layers (53a,53b) have a same thickness as the electrodes (51a,51b). The protective layers which are made of synthetic resin are continuously disposed with the electrodes (51a,51b).
Referring now to the FIG. 2, the brush (54) is slidably in contact with the electrodes (51a,51b) and the protective layers (53a,53b). The brush (54) is not in direct contact with the base plate (50). Each of the electrodes (51a,51b) and the protective layers (53a,53b) have the same thickness. The protective layers (53a,53b) have a function of reducing the friction between the brush (54) and the base plate (50).
However, the conventional throttle opening sensor has drawbacks as hereinbelow. That is to say, the protective layers (53a,53b) do not produce satisfactory antifriction characteristics. When the protective layers (53a,53b) contacts the brush (54), the protective layers (53a,53b) wear out at an early stage. As a result, the brush (54) contacts the base plate directly and the portion of the brush (54) in contact with the base plate (50) wears out. Thus, the structure of the conventional throttle valve sensor can not produce a good durability.